Defect Dipoles Research Articles

Large linear electrostrain of acceptor-donor Co-doped ZnO films

• Piezoelectric coefficient of Fe 3+ -Li + co-doped ZnO thin film is significantly enhanced by constructing acceptor-donor Fe 3+ -Li + ionic pairs in ZnO structure. • The Fe 3+ -Li + co-doped ZnO film has a large linear electrostrain. • The high permittivity has been obtained which is about one order of magnitude larger than that of pure ZnO film. • The piezoelectric performance of the films can be further improved by thermo-electrically treatment. In this paper we present the effective strategy of Fe 3+ and Li + co-doping to enhance the electrostrain of ZnO thin films. Zn 0.88 (Fe 0.06 Li 0.06 )O thin film exhibits an excellent d 33 * value of 415 pm/V and large linear electrostrain of 0.68% after thermal-electric treatment. Both the X-ray diffraction and first principle calculation results have indicated that the Fe 3+ and Li + ions are inclined to adopt a preferential alignment along the edges approximately parallel to [001] direction in the zinc-oxygen tetrahedron and constitute a defect dipole (ionic pairs). It is considered that the local lattice distortion and the electric field-triggered polarization extension and rotation generated by preferential distributed Fe 3+ -Li + ionic pairs are responsible for the outstanding piezoelectric properties and large linear electrostrain.

Ultra–low loss tangent and giant dielectric permittivity with excellent temperature stability of TiO2 co-doped with isovalent-Zr4+/pentavalent-Ta5+ ions

The excellent giant dielectric properties (ExGDPs) are represented in the isovalent–Zr4+/pentavalent–Ta5+ ions co–doped TiO2 with different co–doping percentages (x%ZrTTO). The dopants were dispersed homogeneously in a highly compact–grained ZrTTO microstructure. The mean grain size and cell parameters with bond lengths slightly enlarged as x% increased. The (1%–5%) ZrTTO oxides exhibited ultra–low tanδ values of 0.004–0.016 with the giant dielectric permittivity (ε′∼2.7–3.7 × 104); while the ε′ of the 5%ZrTTO was slightly dependent on the temperature ranging from −60 to 200 °C, following the temperature dependence requirement for application in the X7/8/9R capacitors. Impedance spectroscopy showed a very large resistance of the grain boundaries. The dielectric properties of the 1%ZrTTO were strongly dependent on the applied DC electric field, indicating the dominant internal barrier layer capacitor (IBLC) effect. However, the dielectric properties of the 5%ZrTTO were nearly independent on the applied DC electric field up to 30 V/mm, which was primarily resulted from electron localization in defect dipoles. Therefore, the ExGDPs of the x%ZrTTO were attributed to the combined effects of the IBLC and localized–electron defect–dipoles related to oxygen vacancies (Ti4+⋅e−−VO••−e−⋅Ti4+ and 3Ti4+⋅e−−VO••−TaTi•) and Ti4+⋅e−−TaTi•.

Dielectric properties of BaTiO3-based ceramics are tuned by defect dipoles and oxygen vacancies under a reducing atmosphere

BaTiO3-2 mol% MnYbO3-2 mol% CaZrO3-x mol Nb2O5 (x = 0, 0.3%, 0.5%, 0.7%) ceramics were successfully prepared via the conventional solid‐state method, followed by sintering in a reducing atmosphere. The effect of Nb2O5 doping on the crystal structure, micromorphology and dielectric properties of sintered samples was studied, and the correlation mechanisms between the microstructure and dielectric properties were further determined. For x = 0.5 mol%, optimal dielectric properties with high permittivity (ԑr ∼ 3570), low dielectric loss (tanδ ∼ 0.013) and large insulation resistivity (ρv ∼ 4.95 × 1011 Ω cm) are obtained. On this basis, the temperature coefficient of the capacitance (ΔC/C25°C) ranges between ±15% over the temperature range from −55 °C to 125 °C, satisfying the EIA X7R standard. Analysis shows that the increased permittivity of the doped ceramics at room temperature is due to enhanced short-range hoping polarization triggered by the formation of defect dipoles ([MnTi″−2NbTi·]× and [YbTi′−NbTi·]×) and the release of oxygen vacancies. The high insulation resistivity is related to the grain size and electron migration in the ceramics. In addition, the improvement in the temperature stability of the ceramics is ascribed to a decrease in the Curie temperature. The polarization mechanism presented in this work can be extended to BaTiO3 materials, and the results have important application value for BME-MLCC devices.

A correlative study between the microstructure and electrical properties of lead-free piezoceramics (1-x)BHT-xYLG driven by the synergistic action of multiple factors

Piezoceramics are important functional materials with broad applications in electronics, communication, medicine, and energy. Increasing environmental protection and human health concerns about the toxicity of lead have impelled the development of lead-free piezoceramics. At this work, in combination with phase field simulation, grain growth mechanism, ferroelectric domain wall model, and defect dipole formation mechanism, (1-x)Ba(Hf0.02Ti0.98)O3-x(Y0.5Li0.5)GeO3 [(1-x)BHT-xYLG, x = 0–0.40 mol%] lead-free piezoceramics were designed and fabricated via the two-step synthesis and solid-state reaction method, which yielded superior comprehensive electrical property. Phase field simulation respectively simulates the distribution of ferroelectric domains of O and T phase under different external electric fields (0–20 kV/mm) and the phase states under different temperatures (−100 to 150 °C). This research finds that their electrical properties are affected by the synergistic action of multiple factors, e.g., orthorhombic-tetragonal phase boundary (O-T PB), grain size (GS), domain-wall (DW), and defect dipole (DD), etc., rather than any single factor.

Grain-boundary engineering inducing thermal stability, low dielectric loss and high energy storage in Ta+Ho co-doped TiO2 ceramics

How to achieve a giant dielectric constant and high energy storage density at the same time has been the problem to be solved for donor-acceptor co-doped TiO2 ceramics. In this work, (Ho0.5Ta0.5)0.01Ti0·99O2 - x SiO2, where x = 0, 1, 3, 5 and 7 wt% (HTTO - x wt% SiO2), nanocomposites were prepared via a conventional mixed oxide technique. Significantly, the HTTO - 5 wt% SiO2 composite ceramic exhibits a low dielectric loss (tanδ ∼ 0.012) and an ultrahigh permittivity (εr ∼ 1.29 × 104) at 1 kHz. Also, excellent energy storage property with a high breakdown field strength (Eb ∼1.86 kV/cm) and energy storage density (η ∼ 1.97 mJ/cm3) was obtained in HTTO - 5 wt% SiO2 ceramic. Besides, the enhancement of Eb is attributed to the finer grains and the presence of SiO2 blocking layers in the grain boundaries, which hinder the long-range motion of electrons. It can be concluded that the CP and high energy storage properties arise from the combined contribution of enhanced grain boundary effects and electron-pinning type of defect dipole (EPDD) effects. This study not only proposes an effective method improving Eb, but also offers a new routine for how to simultaneously achieve CP and high η in TiO2 dielectric materials.

Colossal dielectric permittivity in CuCrO2 ceramics

CuCrO2 ceramics were prepared by solid-state reaction method. Low-temperature (10–320 K) dielectric properties were investigated in the frequency range of 102–106 Hz. Colossal dielectric constant behavior composed of two thermally activated relaxations was observed. The low-temperature relaxation (R1) occurring around 125 K with an activation energy of 0.21 eV was ascribed to be a dipolar relaxation caused by defect dipoles of OHO•−Cr3+. The high-temperature relaxation (R2) appearing around 200 K with an activation energy of 0.26 eV was argued to be a Maxwell-Wagner relaxation originating from humidity sensitivity.

High-performance (Na0.5Bi0.5)(Ti0.97Fe0.03)O3-based heterostructure thin films for energy storage capacitors

This work presents a simple process to fabricate Na0.5Bi0.5(Ti0.97Fe0.03)O3/Ba(1–x)SrxTiO3–based heterostructure thin films with a compositional graded sequence, and their energy storage properties are investigated systematically. A simulation technique is used to predict the experimental results. Interestingly, improved energy storage properties are obtained in the “up–graded” film, which is associated with the stress/strain. Furthermore, the “up–graded” film after aging with treating exhibits a higher breakdown strength (EBD = 3176.4 kV/cm), recoverable energy storage density (Wrec = 67.18 J/cm3) and efficiency (η = 75.65%). This can be attributed to the ordered defect dipoles induced by aging with treating driving domain switching, which is favorable for high (Pm–Pr) and large Wrec. These results provide an approach and guidance to design thin film-based devices with optimal energy storage performance.

Optimization of Energy Storage Properties in Lead-Free Barium Titanate-Based CeramicsviaB-Site Defect Dipole Engineering

Optimization of Energy Storage Properties in Lead-Free Barium Titanate-Based Ceramics<i>via</i>B-Site Defect Dipole Engineering

Abnormal dielectric relaxations and giant permittivity in SrTiO3 ceramic prepared by plasma activated sintering

AbstractIn this study, the dielectric properties of SrTiO3 ceramics prepared by plasma‐activated sintering (PAS) were investigated. One of the striking findings is that the material exhibits giant room temperature permittivity (k∼3.5 × 104) and low dielectric loss (∼0.05) at 1 kHz, with the permittivity exceeding that of the conventionally prepared SrTiO3（ST） ceramics (k∼300) by two orders of magnitude. The enhancement of the polarizability was caused by the high concentration of defect dipoles. In this paper, two dielectric relaxation modes of the PAS ceramics below 0°C have been mainly discussed. One dielectric relaxation mode showed higher activation energy than that of the dielectric peak in the same temperature range for the conventional SrTiO3‐based ceramics. This mode was sensitive to humidity, and the strength of this mode was associated with the oxygen vacancies concentration in the ceramics. The other mode exhibited abnormal slowing down of relaxation rate with increasing temperature, which is contrary to the typical dielectric relaxation behavior, and the anomaly persisted over a narrow temperature range. Both modes were observed at the same interface between the grain and grain boundaries.

Deferred Polarization Saturation Boosting Superior Energy-Storage Efficiency and Density Simultaneously under Moderate Electric Field in Relaxor Ferroelectrics

High-temperature dielectric Bi0.5Na0.5TiO3 (BNT)-based relaxors near a morphotropic phase boundary are developed with excellent energy storage performance. Random distribution of polar nanoregions induced by composition modulation would disrupt the ferroelectric long-range dipolar alignment and weaken the coupling between the ferroelectric domains, yielding slender and deferred polarization–electric field hysteresis loops with relatively high saturation polarization. The reversible nano-domain orientation and growth in relaxors under a delayed electric field result in negligible remnant polarization and advantageous energy storage properties. Simultaneously, superior recoverable energy storage density and efficiency are gained, significantly surpassing the state-of-the-art dielectric energy storage materials under similar moderate electric fields. Vacancies, defect dipole behavior, and structural evolution that relied on an electric field and temperature are discussed to disclose the underlying mechanism associated with phase transition. Even thermal stability and large electrostrictive strain with low hysteresis are achieved in elevated temperatures. These features demonstrate the promising candidates for dielectric energy-storage application and provide a strategy in designing relaxors.

The regulation of ferroelectric photovoltaics by non–compensated doping engineering

By absorbing and transferring the photons (nearby bandgap) to photoelectrons, non–compensated (Ce 4+ dopants) doping Bi 5 Ti 3 FeO 15 ferroelectric films present switchable and stable photovoltaics. The defect dipole attraction in non–compensated doping films not only narrows the bandgap effectively by the creation of tunable intermediate sub–band (gap state), but enhances separation of electron–hole pairs in the visible–light region. The concept is proved by dramatic redshift of optical absorbance and intense photovoltaics. Subsequently, contrast investigations of equivalent–compensated doping films offer solid experimental evidences. Furthermore, the intensity of Coulomb attraction between defect dipoles can be destroyed by thermal perturbation, which is empirically supported by the abrupt drop of temperature dependent photovoltage at 340 K. These results demonstrate that non–compensated doping can be a promising route to construct more efficient energy transfer devices.

Structural, dielectric, and magnetic properties of reduced graphene oxide-wrapped MnFe2O4 composites synthesized by in-situ sol-gel autocombustion method

Herein, the reduced graphene oxide (rGO) wrapped MnFe2O4 ([email protected]) composites with different rGO content of 10, 20, 30, and 40 wt% have been synthesized by a one-step in-situ sol-gel autocombustion method. The synthesized composites have been tested for their structural, electrical, dielectric, and magnetic characteristics. The composites are characterized by using standard techniques (XRD, HR-TEM, FTIR, and Raman spectroscopy). The composite having 20 wt% of rGO exhibits the highest value of dielectric constant (ε′∼1.32 × 104 at 100 Hz, ε′∼143 at 1 MHz) and dc conductivity (σdc = 4.31 × 10−6 Ω−1-cm−1) among the investigated composites. The dipole polarization contribution to the dielectric relaxation behavior of [email protected] composites is observed, which arises from the increased vacancy defect dipoles in rGO sheets. The impedance studies show the existence of two different time relaxation phenomena in [email protected] composites. The magnetic measurements reveal the superparamagnetic behavior of composites. The saturation magnetization of composites decreases first with the increase in rGO content up to 20 wt% and then increases with a further increase in rGO content. The findings of this research make these composites a potential candidate for various applications such as EMI shielding and energy storage devices.

Structural Modifications and Electromagnetic Property Regulations of Ti3AlC2 MAX for Enhancing Microwave Absorption through the Strategy of Fe Doping

AbstractMicrowave absorption enhancements through atomic doping in traditional materials remain a significant challenge. Herein, the MAX phase materials of Fe‐doped Ti3AlC2 (xFe‐Ti3AlC2, abbreviated as TFAC‐x, x = 0, 0.2, 0.3, 0.4) are synthesized via solid‐phase reaction method. The X‐ray diffraction, Raman, and scanning electron microscopy confirm that the TFAC‐x samples are of high quality with multiphase coexistence structure and abundant interfaces. Relying on the improved impedance matching originating from structural modulation, the synergistic effects of dielectric loss and magnetic loss, as well as the enhanced interfacial polarization and defect dipole polarization, both the microwave absorption intensity and bandwidth are greatly strengthened. Accordingly, the minimum reflection loss of TFAC‐3 sample can achieve ‐33.3 dB and the absorption bandwidth could reach 3.9 GHz with the thickness of only 1.5 mm. Moreover, the investigated frequency can cover 86% through regulating their thickness from 1.5 to 5 mm, showing the excellent frequency modulation effect. The revealed improvements in both the microwave absorption performance and properties of TFAC‐x may enlighten the designing of microwave absorption materials and have potentials for widespread applications.

Capturing Carriers and Driving Depolarization by Defect Engineering for Dielectric Energy Storage.

The inevitable defect carriers in dielectric capacitors are generally considered to depress the polarization and breakdown strength, which decreases energy storage performances. Distinctive from the traditional aims of reducing defects as much as possible, this work designs (FeTi' - Vo••)• and (FeTi″ - Vo••) defect dipoles by oxygen vacancy defect engineering in acceptor doped Sr2Bi4Ti(5-x)FexO18 layered perovskite films with n-type leakage conductance. It is shown that oxygen vacancies effectively capture electrons (carriers) in n-type dielectrics to enhance the breakdown strength. Meanwhile, defect dipoles provide a driving field for depolarization to engineer the generation energy of domains and the domain wall energy, which effectively lowers the residual polarization Pr but not at the expense of the maximum polarization Pmax as relaxor ferroelectric regulations. Such defect engineering effectively breaks through the limitation, in which the energy storage density suffers from the trade-off relationship between polarization and breakdown strength. The Sr2Bi4Ti4.92Fe0.08O18 film with the proper oxygen vacancy content achieves a high energy density of 110.5 J/cm3 and efficiency of 70.0% at a high breakdown strength of 3915 kV/cm. This work explores an alternative way for breakthroughs possible in the intrinsic trade-off relationship to regulate dielectric energy storage by defect engineering.

Microwave dielectric properties of Al-doped Ba4(Sm,Nd)9.33Ti18O54 ceramics added with TiO2 and sintered in oxygen

TiO2-added Ba4(Sm0.65Nd0.35)9.33Ti17.75Al0.33O54 (BSNTA) ceramics were fabricated with a solid-state reaction method. Slight TiO2 addition increased the dielectric constant (εr) of BSNTA ceramics and brought about a near-zero temperature coefficient of resonant frequency (τf). Sintering in oxygen increased the density of TiO2-added BSNTA ceramics and surprisingly concurrently improved the εr and Q × f value. Thermally stimulated depolarization current (TSDC) measurements indicated that TiO2 addition introduced oxygen vacancies (VO..) and TiTi'−VO.. defect dipoles and that an oxygen-sufficient sintering atmosphere decreased the concentration of these defects and thus increased Q × f values. From the viewpoint of defect behaviors, this observation offered original insight into the mechanism by which the sintering atmosphere had impacts on the Q × f value of titanate ceramics. Satisfying dielectric properties (εr = 79.5, Q × f = 14,840 GHz, τf = 0 ppm/°C) were attained in oxygen-sintered BSNTA-0.6%TiO2 ceramics that might be suitable for the manufacture of miniature microwave devices such as filters and antennas.

Defect dipole induced improved electrocaloric effect in modified NBT-6BT lead-free ceramics

The Rietveld refinement of the polycrystalline powders of 1% Fe and Mn-doped (Na0.5Bi0.5)0.94Ba0.06Ti0.98V0.02O3 at the Ti-site confirmed a single rhombohedral (R3c) phase. The bandgap, (Eg) was affected by the anti-phase octahedral tilt angle and the spin-orbit splitting energy of Ti4+2p3/2 and Ti4+2p1/2 states. The decrease in Bi loss and increase in the binding energy of Ba due to Fe/Mn doping has been correlated to the strengthening of Bi-O and Ba-O bonds which was revealed from the XPS studies thereby further related to the average A-O bond length from structural studies. Hence, a reduction of oxygen vacancy (VO) for the doped samples has been justified. A significant improvement of the dielectric constant, relaxation time (τ0), and the decrease in conductivity due to doping was revealed from the frequency-dependent (10 Hz-1 MHz) dielectric measurement study. The conduction and relaxation process are dominated by the short-range movement of defects. The activation energy (Ea~1 eV) revealed that there is a presence of double-ionized VOs. The ECE study showed a significant enhancement of the changes in entropy, ΔS, and the adiabatic temperature difference, ΔT, due to doping, with ΔT being highest in the Fe-doped sample. Such improvement of dielectric and ECE properties was confirmed due to the reduction of the mobility of oxygen vacancy because of the formation(MnTi′′−Vo••)/(MnTi′−Vo••)• and FeTi′′−Vo••/(FeTi′−Vo••)• defect dipoles.

Colossal permittivity and ultralow dielectric loss in SrTi1-xNbxO3 ceramics sintered at different atmospheres via defect chemistry improvement

SrTiO3 is recognized as one of the most promising candidate materials for colossal permittivity. However, the origin of colossal permittivity is still not well understood. In this work, SrTi0.998Nb0.002O3 ceramics (SNT) were synthesized via typical solid-state reaction process and sintered in air, N2, and N2–H2 atmospheres (labelled as SNT-Air, SNT-N2 and SNT-H2). The SNT-H2 ceramic achieves a colossal permittivity of 89254 and a dielectric loss of 0.013 at 1 kHz and maintains a colossal permittivity (∼8 × 104) with a low dielectric loss (<0.1) throughout a wide temperature (50 °C–300 °C) and frequency (100 Hz-1 MHz) range. The mechanism of the outstanding properties of SNT-H2 ceramics was investigated by X-ray photoelectron spectroscopy (XPS), impedance spectrum and activation energy analysis. The TiTi′−VO⋅⋅−TiTi′ defect dipole and VO⋅⋅−3TiTi′−NbTi⋅ defect dipole clusters exist in SNT ceramics by sintering in the reducing atmosphere of H2. Otherwise, the grain is semi-conducting, and the grain boundary is insulating in SNT-H2 ceramics by impedance spectrum analysis. In conclusion, the defect dipole and clusters localized in the grain could be the fundamental cause for the ultra-dielectric properties. As a result, The SNT-H2 ceramic exhibits the colossal permittivity with low dielectric loss, as well as good temperature and frequency stability, potentially aiding in the development of colossal materials.

High energy storage performance in tungsten bronze-based relaxor ceramic via doping with CuO

Developing high-performance lead-free ceramics for capacitive applications has become more and more critical owing to the increasing concern about environmental protection and electronics industry. Here, a dual defect manipulation strategy was proposed to boost energy storage performance in CuO-modifided Sr2NaNb5O15-based lead-free tungsten bronze relaxor ceramic. Induced defect dipoles and reduced oxygen vacancy concentration generate a pinched polarization hysteresis loop and high breakdown strength, thus giving rise to a high recoverable energy density of 4.17 J/cm3 and efficiency of 89.5% at 350 kV/cm, together with an ultrafast discharge speed of 55 ns and a high power density of 118.4 MW/cm3. This proposed strategy is generalizable for realizing high capacitive performance in tungsten bronze-based dielectrics and other lead-free ceramics that benefit from dual defect manipulation.

A Flexible Sandwich Structure Carbon Fiber Cloth with Resin Coating Composite Improves Electromagnetic Wave Absorption Performance at Low Frequency.

In order to improve the electromagnetic wave absorbing performance of carbon fiber cloth at low frequency and reduce the secondary pollution caused by the shielding mechanism, a flexible sandwich composite was designed by a physical mixing coating process. This was composed of a graphene layer that absorbed waves, a carbon fiber cloth layer that reflected waves, and a graphite layer that absorbed transmitted waves. The influence of the content of graphene was studied by a control variable method on the electromatic and mechanical properties. The structures of defect polarization relaxation and dipole polarization relaxation of graphene, the interfacial polarization and electron polarization of graphite, the conductive network formed in the carbon fiber cloth, and the interfacial polarization of each part, combined together to improve the impedance matching and wave multiple reflections of the material. The study found that the sample with 40% graphene had the most outstanding absorbing performance. The minimum reflection loss value was −18.62 dB, while the frequency was 2.15 GHz and the minimum reflection loss value compared to the sample with no graphene increased 76%. The composites can be mainly applied in the field of flexible electromagnetic protection, such as the preparation of stealth tent, protective covers of electronic boxes, helmet materials for high-speed train drivers, etc.

Enhanced photovoltaic performance of SnO2 based flexible perovskite solar cells via introducing interfacial dipolar layer and defect passivation

Low processing temperature of tin dioxide (SnO 2 ) is essential for flexible perovskite solar cells (f-PSCs). However, the low processing temperature of SnO 2 results in an inferior photovoltaic performance due to compromised carrier transport capacity in devices. The deteriorated carrier transport mainly results from the suppressed interfacial charge transfer. To overcome these issues, we used ionic liquid to modify the interface between perovskite films and SnO 2 in f-PSCs. Density Function Theory calculations were implemented to demonstrate the ionic liquid modification induced the formation of interfacial dipolar layer (IDL) and defect passivation in perovskites. The IDL and defect passivation improves the interfacial charge transfer and intra-perovskite charge transport, respectively. As a result, the f-PSCs based on ionic liquid modified SnO 2 yield the champion power conversion efficiency (PCE) of 19.0%. This work provides a method to effectively improve the performance of f-PSCs using low-temperature processed SnO 2 ESLs using affordable materials and a facile approach. • Ionic liquid is used to form an interfacial dipole layer (IDL). • Ionic liquid can also passivate defects in perovskite. • IDL and defects passivation contribute to the performance improvement of f-PSCs. • All fabrication processes of f-PSCs are conducted at the low temperature (≤100 °C). • Power conversion efficiency of f-PSCs is improved from 13.5% to 18.1%.